Project Summary Many bacterial infections are caused by diverse microbial communities. These polymicrobial infections often result in higher bacterial burdens and more severe disease than single-species infections, a phenomenon called ?synergy?. While synergy between bacteria has been recognized for over a century, the molecular mechanisms that cause it have proven difficult to elucidate. My laboratory has focused on synergistic interactions between the opportunistic oral pathogen Aggregatibacter actinomycetemcomitans (Aa) and the human oral commensal bacterium Streptococcus gordonii (Sg). In the previous grant cycle, we showed that synergy in Aa/Sg co- infections is caused by several processes involving Aa's utilization of, and response to, the Sg metabolites L- lactate and H2O2. The goal of this renewal is two-fold: 1) to determine how the Aa-Sg synergy mechanisms that we have discovered impact robustness to environmental disturbances and to determine precisely how the spatial organization of these species impacts such interactions during infection (Specific Aims 1 & 2); and 2) to utilize high-throughput genomics to define the metabolic interactions that occur between Aa and a range of co-infecting bacteria (Specific Aim 3).